As is well known in the art, a brushless synchronous generating system typically includes a permanent magnet generator (PMG), an exciter and a main generator. The PMG develops an alternating current which is rectified to energize the exciter field, inducing an AC current in the exciter armature. The exciter armature current is rectified to energize the main generator field, inducing an AC current in the main generator armature.
The brushless generating system can also be operated as an induction motor, as for starting an aircraft engine. A source of starting power can be external electrical power from an auxiliary power unit (APU) or another generator being driven by a previously started engine.
When the generating system is operating as a synchronous generator, a relatively large suppression resistor is required across the main generator field to suppress a voltage induced across the main generator field resulting from rectifier switching.
When the generating system is operating as an induction motor, an AC current is induced in the main generator field. Because the rectifier coupling the main generator field and the exciter conducts only during the half cycle that it is forward biased and the suppression resistor conducts during the half cycle that the rectifier is reversed biased, a pulsating DC current is developed in the main generator field.
In order to start the machine as an induction motor, however, an alternating current must circulate through the main generator field. Further, the suppression resistor is typically not sized sufficiently to dissipate the power during the half cycle that the rectifier is reverse biased and not conducting. Therefore, an alternate current path is necessary to pass the induced current when the generating system is operated as an induction motor. The rectifier must also be protected from a high reverse bias voltage induced across the alternate current path.
The present invention is directed to overcoming one or more of these problems.